Expansion vessel of a boiler: role and utility


An essential element for the proper functioning of a boiler installation, its role is both to absorb the variation in the volume of water in the event of a change in temperature, but also to prevent wear and tear on the heating circuit by maintaining constant pressure. In this article, discover the role and usefulness of the expansion tank, but also our advice for adopting the right actions during a breakdown.

What is the role of the expansion tank?

Concretely, when your boiler is running, the water contained in the heating circuit expands causing a variation in the volume of water under the effect of temperature. To avoid the danger posed by this pressure, the expansion tank absorbs the excess water, thus preventing a negative pressure during cooling. Whether you are in an apartment or a house, it is necessary to have your boiler checked once a year by a professional. During its passage, it will make sure that there is sufficient air in the expansion tank. Maintaining your boiler is therefore dispensable to prevent a certain number of inconveniences.

Operation of the expansion vessel

The expansion tank has the shape of a sphere or a metal cylinder. It is made up of two sections separated by a waterproof membrane or by a bladder. The connection to the water network is made from the top. The installation placed on the water return allows the water overflow to be evacuated.

At the bottom of the expansion vessel, a valve specially dedicated to inflating or re-inflating the gas envelope makes it possible to avoid overpressure and / or to manage the correct pressure depending on the model of the boiler and its installation.

The objective is to have constant pressure guaranteeing a lasting installation.

In all cases, a professional must ensure that various parameters are correctly adjusted in order to avoid any risk of breakdown. In particular, he will make sure, using a manometer, that the pressure indicated in bar is sufficient. This annual maintenance is essential to prevent serious damage such as corrosion or the rupture of welds.

What are the different types of expansion tank?

There are different models. Open or closed expansion vessels, at constant or variable pressure. The most widely used are closed expansion vessels with variable pressure. They either have a membrane or a bladder. For both, the base part of the expansion tank is filled with the excess volume of water in the circuit while the upper part is filled with compressed air.

The arrival of water in the vase pushes the air part which compresses, absorbing the overflow of pressure. Thus, the pressure remains stable. Likewise, the expansion tank will avoid a depression when the water cools.

Older boiler systems are fitted with open or closed constant pressure expansion vessels. These models are currently not recommended.

The systems with an open model have a direct impact on the corrosion of the materials while the closed installations with constant pressure use a valve to evacuate the excess water.

How to react when your expansion tank breaks down?

Detect expansion tank failure

Usually, when the water expands and the pressure increases, the boiler goes into lockout. Before that, several indicators can alert on an upcoming failure such as, for example, a frequent and regular drop in pressure or the display of an error.

For the oldest boilers, these functions do not exist, they release in small quantities, the surplus water, by the means of a safety valve.

At the slightest sign, it is preferable to approach your installer for an intervention. You can also try to re-inflate your expansion tank yourself. For this, you will need the necessary equipment (compressor, pressure gauge, etc.). Be careful, however, a deflated expansion vessel is as dangerous as one that is over-inflated. Note, the ideal pressure is between 0.8 and 1.5 bar.